1. Field of the Invention
The present invention relates to a method for purifying an exhaust gas, and more specifically, it relates to a method for efficiently purifying the exhaust gas by removing carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) in the exhaust gas.
2. Description of the Related Art
In the past, there have been known as catalysts for purifying automotive exhaust gas many three-way catalysts for purifying exhaust gas which purify exhaust gas by simultaneously oxidizing the carbon monoxide (CO) and hydrocarbons (HC) in the exhaust gas and reducing nitrogen oxides (NOx) in the exhaust gas. Such typical catalysts are comprised of, for example, cordierite or other refractory carriers coated with a .gamma.-alumina slurry, calcined, then made to carry a noble metal such as Pd, Pt, and Rh.
The performance of such an exhaust gas purifying catalyst, however, largely depends upon an air-fuel (A/F) ratio set for the engine. In the case of a lean air-fuel mixture, that is, at the lean side with a large air-fuel ratio, the amount of oxygen in the exhaust gas after combustion increases, and therefore, the oxidation action becomes more active and the reduction action becomes inactive. Conversely, at the rich side with a small air-fuel ratio, the amount of oxygen in the exhaust gas after combustion decreases, and therefore, the oxidation action becomes inactive and the reduction action becomes more active. On the other hand, in recent years, automobiles have been made to be driven at the lean side as much as possible during ordinary driving so as to burn an oxygen rich air-fuel mixture and thereby meet with the demands for lower fuel consumption. A catalyst which can sufficiently remove NOx even at the lean side has therefore been desired.
The present inventors, under these circumstances, proposed as a catalyst for purifying an oxygen rich exhaust gas (i.e., an exhaust gas containing oxygen in an amount exceeding the amount of oxygen necessary for completely oxidizing carbon monoxide, hydrocarbons, hydrogen and other reducible substances contained in the exhaust gas), a catalyst which would adsorb or absorb NOx at the time of an oxygen rich (i.e., lean) atmosphere and would reduce the nitrogen oxides (Nox) at the time of stoichiometric state or rich state and simultaneously perform the oxidation of the carbon monoxide (CO) and hydrocarbons (HC), for example, a Pt/Ba/Al.sub.2 O.sub.3 catalyst comprised of platinum and barium supported on an alumina carrier (see Japanese Patent Application No. 4-130904). This catalyst, however, did not have a sufficient NOx purification rate after extensive use, even with an increase in the amount of the platinum and barium contents, and therefore, development of a practical exhaust gas purifying catalyst with a high durability has been desired.
As mentioned above, there are cases where a Pt/Ba/alumina catalyst lacks a practically sufficient NOx removal rate after extensive use. This is because the sulfur dioxide (SO.sub.2) present in the exhaust gas to be treated, generated by the combustion of S in the fuel, reacts with the Ba and converts it to BaSO4, which does not have the ability to adsorb or absorb NOx, whereupon the NOx removal rate is decreased. This phenomenon becomes marked in particular when performing high temperature durability tests.